The present invention relates to noise detection and, more particularly, to estimation and cancellation of noise in a system or device.
Electronic signals are often composed of a desired signal and noise. The noise may be considered anything undesirable that is included in the electronic signal. At times, the noise may interfere with the interpretation of the desired signal within the electronic signal. In order to interpret the desired signal with greater accuracy, the noise should be removed from, or reduced in, the electronic signal without significantly degrading the desired signal.
One example of a device that receives an electronic signal is a modem. A modem typically modulates information for transmission over a communication channel and demodulates signals received from the communication channel to recover information. Noise may corrupt the modulated signal, thereby degrading the desired signal, thereby requiring the use of noise reduction techniques to recover the desired signal. Other devices which are sensitive to noise include medical devices such as EKG monitors, audio systems, and industrial devices. For example, even though an electronic device is internally well-buffered, such as in the case of an EKG monitor, external power line noise can filter into the system, interrupting the desired signal. As another example, where a machining device has motion control as part of its feedback, the motion control may experience interference from power line noise due to the harsh environment of the machining device.
Moreover, some devices use power lines as the medium by which to send data. The interference from the power line in that instance may significantly interfere with the data communication. Further, other types of electronic signals, such as video signals, may experience a "hum" due to power line noise. In the case where the video signal must be precise, the "hum" may interfere with the video.
Noise in a system may be either periodic or non-periodic. One source of periodic noise is the interference from AC power lines. Power line noise is commonly either 50 Hz or 60 Hz, depending on the power convention for the particular country. This type of noise may effect an electronic device in various ways. A high speed modem, for example, may suffer from periodic power line interference from one of several sources. There could be power line noise carried on the phone line to which the modem is connected. There could also be noise from an insufficiently filtered power supply for either the modem itself or the host computer to which the modem is attached.
There are various approaches to reduce noise, and in particular periodic noise, on a system. Two known approaches involve filtering the noise. First, a simple high pass filter may be used. The filter is designed to pass frequencies in the range of the desired signal and to block other frequencies. Thus, this approach may not require that the filter be designed specifically for a known frequency (such as in the power line example of 50 Hz or 60 Hz). In the alternative, a narrow notch filter may be designed if one has a priori knowledge of the frequency of the periodic interference. Both of these approaches may be useful if the frequency of the periodic noise does not fall within the range of the desired signal.
In those instances that require knowledge of the frequency of noise, a method and apparatus may be required for determining the frequency of an interfering periodic signal. Techniques exist which are known in the art for estimating frequencies of periodic signals on a communication channel. These techniques often involve generating a power density spectrum from a data sequence, and then analyzing the spectrum to identify peaks. While these techniques are useful for analysis of an arbitrary spectrum, they may require a large amount of computation. It is desirable to minimize the computational demands of the noise reduction technique. Another known approach is to filter the data sequence using narrow bandpass filters, with one filter centered at each frequency in question, and then compare the amplitudes of the filter outputs. This may be simpler than generating the complete spectrum, but a simpler approach is still desirable.
Once the frequency of the periodic noise is determined, the noise should be reduced or eliminated. In the power line example, desired signals (audio, data communications, etc.) are often corrupted by periodic interference such as from 50/60 Hz power line noise. Various techniques for removing the interference from the signal have been developed, such as high-pass filtering, notch filtering, and adaptive noise cancellation techniques. These techniques are often inadequate for a number of reasons. First, the interference often contains harmonics that exist in the same frequency range as the signal, in which case high-pass filtering is not sufficient. Second, the frequency of the interfering signal may not be known exactly, in which case a notch filter is not acceptable. Third, adaptive noise cancellation techniques typically require a reference which consists of the interference alone without the signal. In many cases, such a reference is unavailable. Fourth, the noise cancellation techniques often treat the periodic noise and the non-periodic desired signal in the same manner causing, in many instances, the desired signal to degrade when attempting to eliminate the noise within the system. For example, a notch filter, which is set at 60 Hz, treats the periodic noise and any desired signal component at 60 Hz equally, so that the desired signal may also be degraded.
A high speed modem is one example of a device that may suffer from periodic interference. Such interference will likely have harmonics throughout the frequency range of the modulated signal. As a further complication, the frequency of the periodic interference may be known approximately but not exactly. In addition, there will typically not be an interference reference available. Accordingly it is desirable to have an improved frequency estimator and interference cancellation method and apparatus.